Squares are not adjacent if there is a wall or closed door between them. From Page 2 of the rules
Any reference in the game to Adjacent spaces means those spaces that
are up, down, left, or right from a space. Diagonal spaces are not
Adjacent. Closed Doors and Walls prevent spaces from being Adjacent –
unless the Wall segment is Destroyed. A Wall ...

Crew Change lasts until you decide to change roles again. You don't automatically change back to your old role. The "cost" is having to get back to the truck, finish the turn, and then spend the 2 AP to change.

Placing Smoke on a POI
Yes, you put the smoke on the same space as the POI, and the POI is unaffected. If the smoke later turns into fire, the POI will then be removed.
Placing a POI on Smoke
In the family version (page 6 of the rules), yes. They even eliminate the smoke for you!
If the Target space has a Fire or Smoke marker, remove that Fire/Smoke
...

This is just off the top of my head, but if the issue you're having is that someone needs to be 'stuck' there, then how about giving everyone a special action to skip their whole turn to "radio the driver" to take the action. This way, effectively, any player can be it without having to be in the truck, and people can take turns. Try it out, and if it ...

This seems to be pretty explicit in the version of the rules I have, found here. Under Knocked Down in the Experienced Rules, it very specifically states:
When a Firefighter is Knocked Down, the Firefighter is placed in one of the two spaces of the Ambulance's current Parking Spot.
My interpretation of this is that the Firefighter has to go to where ...

No, blank markers are "False Alarms", not "Victims". They do not count towards the loss condition of "4+ Victims Lost", just like you place revealed False Alarms on the Rescued space but they do not count towards the victory condition of "7 Victims Rescued".

The Action Point (AP) cost is unchanged. From page 4 of the rule book
A player gets 4 Action Points (AP) to spend on their turn. Each Action has a corresponding AP cost to perform. A player may select from any of the available Actions in any order and spend the appropriate AP to perform it. An action may be performed more than once per turn, so long as ...

While I haven't found an official answer to this question yet, I played the game some more and it seems to me that it is more in the spirit of the game to leave the hot spot in place, even after putting the flames out.
Considering that there can only be a limited number of new hot spots (placed via flare-ups) - 6 for all difficulties except Heroic, where ...